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Modules/_threadmodule.c:52:47: runtime error: signed integer overflow: 2387971499048 + 9223372036000000000 cannot be represented in type 'long'

Looks like this is what my thread.patch was fixing in <https://bugs.python.org/issue1621#msg271057>. You’re welcome to use my patch, but I won’t have time to work on it myself.

> Modules/_threadmodule.c:52:47: runtime error: signed integer overflow: 2387971499048 + 9223372036000000000 cannot be represented in type 'long'
How do you reproduce the issue? The thread module should limit the maximum timeout to PY_TIMEOUT_MAX. Maybe PY_TIMEOUT_MAX is too big?

I've created a PR based on Martin Panter's patch.

Victor, if you run the test suite, one of the test cases should trigger the overflow. I used to compile with Undefined Behaviour Sanitizer to print messages when these errors occur; see <https://bugs.python.org/issue1621#msg271118> for my setup at the time. I presume Antoine did something similar.
I do not remember, but suspect the test case might be the following lines of "BaseLockTests.test_timeout" in Lib/test/lock_tests.py, testing a fraction of a second less than PY_TIMEOUT_MAX:
# TIMEOUT_MAX is ok
lock.acquire(timeout=TIMEOUT_MAX)
Perhaps reducing PY_TIMEOUT_MAX by a few centuries would be one way to avoid the problem. In my patch I avoided the problem by rearranging the arithmetic, so that the timeout value is only compared and reduced, never added.

In short, a+b can overflow, but a-b cannot?

> In short, a+b can overflow, but a-b cannot?
I think it's more that by always checking the elapsed time against `now() - starttime`, you never need to represent the time at which the timeout should happen - which may be so far in the future that it causes a signed overflow.

I think PR https://github.com/python/cpython/pull/28674 has resolved this issue.

> I think PR https://github.com/python/cpython/pull/28674 has resolved this issue.
You're right.
_threadmodule.c now uses _PyDeadline_Init() which calls _PyTime_Add(), and _PyTime_Add() prevents integer overflows; Extract of its implementation:
// Compute t1 + t2. Clamp to [_PyTime_MIN; _PyTime_MAX] on overflow.
static inline int
pytime_add(_PyTime_t *t1, _PyTime_t t2)
{
 if (t2 > 0 && *t1 > _PyTime_MAX - t2) {
 *t1 = _PyTime_MAX;
 return -1;
 }
 else if (t2 < 0 && *t1 < _PyTime_MIN - t2) {
 *t1 = _PyTime_MIN;
 return -1;
 }
 else {
 *t1 += t2;
 return 0;
 }
}